This invention relates to vacuum pumps and, more particularly to oil-free or dry mechanical vacuum pumps.
Oil free or dry vacuum pumps, ie those having an oil free swept volume are well known and are extensively used to evacuate enclosures in clean environments such as those found in the semiconductor industry.
Typical pumps of this type can comprise a chamber having one or, more commonly, a plurality of, for example four, pumping stages, each containing inter-meshing pairs of rotors to effect a pumping action and urge gas being evacuated from a pump inlet, through the pumping stages in turn, and subsequently urge the gas out of the pump through a pump outlet.
One rotor of each pair is attached to a first shaft passing through the stage(s) with the second rotor of each pair being attached to a second shaft also passing through the stage(s). One of the shafts is driven by a motor and the other is usually driven synchronously in the opposite direction by means of timing gears attached to the respective shafts.
The rotors of each pair are commonly of the "Roots" type or of the "Northey" (or "Claw") type, both well known per se in the vacuum pump industry. In multi-stage pumps, each stage may possess the same type of rotor pairs or there may be different types of rotor pair in different stages. For example, one commercial vacuum pump sold by the Applicants comprises a first stage possessing a "Roots" type rotor pair and the second, third and fourth stages all possessing "Claw" type rotor pairs.
Alternative pumps of this type are regenerative pumps in which a disc shaped rotor attached to a shaft is driven at high speed by an electric motor usually positioned about the shaft. The rotor has a plurality of teeth on its edge or arrayed on one or both of its faces and, in use, the teeth rotate within passageways in a pump stator and urge molecules of gas being pumped through the passageways. At one place in the passageway, a stripper deflects the molecules in to the next passageway or to a pump exhaust.
A gearbox is usually positioned at the driven end of the shaft(s) containing the shaft end(s), bearings within which the shaft(s) rotate, any timing gears and commonly the motor positioned about the driven shaft. Further bearings may optionally be present at the opposite (non-driven) ends of the shaft(s).
For reasons of cleanliness and non-contamination of the gases being pumped to avoid in particular the possibility of transfer of such contamination back in to the enclosure being evacuated, the oils and/or greases necessarily associated with the gearbox need to be contained and isolated within the gearbox.
For the same reasons, the enclosed gearbox is normally positioned adjacent the pump stage associated with the pump outlet. However, for practical engineering reasons, the gearbox cannot be fully isolated, in particular because of the slight leakage always associated with shaft seals which need to be present about the shafts and attached to a head plate between the gearbox body and the pump stages. This is particularly true for seals of the non-contacting type which are often used to minimise power consumption or because the speed of shaft rotation is too high for contact seals such as lip seals.
As such, a common problem with some designs of vacuum pump is that the gearbox shaft seals at the exhaust end of the pump tend to be exposed to different pressures depending on the pump duty. The seal pressure is typically some intermediate pressure between that of the inlet and outlet of the final pump chamber. During pump "roughing" condition this will tend to be greater than atmospheric pressure, whereas at ultimate vacuum condition this will tend to be lower than atmospheric pressure. This can lead to a certain flow of gas past the shaft seals, ie in to or out of the gearbox respectively. This can be particularly the case with non-contacting shaft seals where this may have the undesirable effect of carrying contamination in to the gearbox and oil/lubricant mist out of the gearbox.